1. Field of the Invention
This invention relates to the field of analog integrated circuits in general and circuitry for improving the linear operating range and the signal to noise ratio (SNR) of differential bipolar transconductance elements in particular.
2. Background Art
A transconductance element is a circuit element that converts a voltage input into a current output. The output of a transconductance element is dependent on its gain, measured in units of "conductance." The gain of a transconductance element is determined by the ratio of its output current, I.sub.O, to the input voltage, V.sub.IN. This ratio I.sub.O /V.sub.IN is called the "transconductance" and is referred to as g.sub.m.
Typically, a transconductance element has two voltage inputs, V.sub.1 and V.sub.2. In so-called "normal" mode, the input voltages are changed differentially, independent of each other. In "common" mode, both input voltages change levels together. Common mode transconductance elements are often used as linear amplifiers. A measure of the performance of a transconductance element operating as a linear amplifier is the "common mode rejection ratio" (CMRR).
The differential output of a transconductance element-based linear amplifier is given by: ##EQU1## where I.sub.O is equal to the differential transconductance g.sub.md multiplied by the difference between input signals V.sub.1 and V.sub.2 plus the common mode transconductance g.sub.mcm multiplied by the sum of V.sub.1 and V.sub.2 divided by two. Ideally, it is desirable to suppress the common-mode component, (a non-linear component). Therefore, to achieve linear gain from differential pairs, they are designed so that the ratio of the differential gain to the common-mode gain, the CMRR, is as high as possible. CMRR is usually specified in decibels. The common-mode input range is the voltage level over which the inputs may vary.
It is desirable that differential amplifiers be easily tunable, be simple in design to avoid excess phase effects caused by unavoidable parasitics, and be linear over a large input voltage range to guarantee a high dynamic range. Fully-differential circuits are used for their higher power supply rejection ratio (PSRR) and CMRR.
However, fully-differential circuits often suffer from a limited linear operating range. An example of a typical fully-differential circuit suffering this drawback is the emitter-coupled pair. A small range in linearity implies that the circuit can receive input voltage values only over a small range before resulting in non-linear outputs. Also, many designs of fully-differential circuits do not have a high signal-to-noise ratio level. Having a low equivalent noise resistance in amplifiers increases the performance of the amplifier by preventing the signal from being obstructed by noise.
A number of prior art transconductance-based amplifiers have been developed. These include emitter-coupled pair, series diode linearization, transistor ratio linearization, two differential pairs with ratios in parallel, and amplifiers using level shift techniques. These prior art amplifiers are described below.